Fix broken links to user page URLs in certain cases.

When Review Board is installed *not* at the root of a server, say,
/reviewboard/ , the user links are broken. This happens in many places
where User Links are obtained, for e.g. a) when searching, b) change
description links when a user is changed in the Review Request.

The application tries to use Django's User.get_absolute_url(), which
always returns /users/<username> ignoring the application path. This
patch fixes this in 2 places

a) Adding a serialize_url_field to UserResource. This would fix all API
   calls

b) Adding a ABSOLUTE_URL_OVERRIDES section in settings.py for auth.user.
   (https://docs.djangoproject.com/en/1.4/ref/settings/#absolute-url-overrides)

We could have skipped (a) above by just adding ABSOLUTE_URL_OVERRIDES
alone as djblets' serialize_url_field calls User.get_absolute_url and
(b) would have solved it, but this doesnt take  localsites into
consideration. Actually (b) above doesnt work with localsites, but then
I found that localsites are not very well handled in other pages as
well. For e.g. in the dashboard of a localsite, if you try to search for
reviewrequests for a particular localsite, it doesnt show that because
the url being searched is only /api/search, not s/localsite/api/search
(If I run the search for s/localsite/api/search, it returns results
correctly). I will file a separate bug for that , but once that is
fixed, the solution (a) above is more correct because it takes localsite
into consideration.

This patch should fix problems for all users who dont use localsites.

Reviewed at http://reviews.reviewboard.org/r/3836/